The following relates to the printing arts. It finds particular application in spatial alignment of highlight and black toner development systems in black xerographic printing with highlighting, and is described with particular reference thereto. The following finds more general application in spatial alignment of xerographic marks produced by different toner development systems such as are used in full color xerographic printing, two-tone xerographic printing, and so forth.
In xerographic printing employing a single toner development system, a moving photoreceptor belt passes through a charging station where it is electrostatically charged. The electrostatically charged belt then passes through an imaging station where an electrostatic image is formed on a portion of the belt by selectively discharging regions of the photoreceptor belt to form a latent image. The selective discharging is typically performed by selective exposure to visible, infrared, ultraviolet, or other light, although other spatially selective electrostatic discharge systems can be used. The electrostatic latent image is developed at a developing station where toner material selectively coats the latent image based upon the local electrostatic charge, thus forming a toner image corresponding to the latent image. At a transfer station, the toner image is transferred by contact to paper or another print medium. After leaving the transfer station, the belt portion containing the toner image passes through a cleaning station that removes residual toner to erase the toner image, and the belt portion then passes back into the charging station to begin processing for another page. The paper or other print medium, after leaving the transfer station, passes through a fuser which applies pressure and heat to fuse the toner to produce the final image on the paper or other print medium.
Some types of xerographic printing systems include multiple toner development systems. For example, full color CMYK xerographic printing systems typically include cyan (C), magenta (M), yellow (Y), and black (K) toner development systems. As another example, a black printing system may provide a primary black (K) toner development system and also a highlighting color (e.g., red) toner development system for providing selected highlighted marks distinct from the general black coverage.
When multiple toner development systems are employed, each toner development system typically includes its own charging station, imaging station, and development station. The moving photoreceptor belt successively passes through the multiple toner development systems to acquire a combined toner image including multiple superimposed toner images produced by the multiple toner development systems. The combined toner image passes through a transfer station to be transferred to the paper or other print medium, then through a cleaning station to remove residual toner, and back to the starting toner development system to begin processing for another page.
For good image quality, the marks produced by the different toner development systems should be accurately relatively spatially registered on the photoreceptor belt. That is, the superimposed toner images from the multiple toner development systems should be accurately aligned with each other.
Heretofore, registration of the superimposed toner images has been performed in various ways. In a manual approach, calibration sheets are printed with, for example, neighboring black regions and highlight color regions. The spacing of these printed regions on the calibration sheets is compared with the intended spacing, and adjustments are made at one or more of the imaging stations. Additional calibration sheets are printed and manually re-measured, and this process is repeated until the spacing of the black and highlight regions on the calibration sheets matches the intended spacing. Such manual approaches are time-consuming, waste paper or other print media, and are not amenable to rapid periodic registration calibration during printing. Moreover, intervening processes such as toner spreading during the fusing can limit the accuracy of the manual spacing measurements used for the manual registration process.
Another approach is the so-called “marks-on-belt” or MOB process. In this approach, a toner development system to be aligned generates slanted-bar or chevron toner marks on the photoreceptor belt. A dedicated MOB sensor detects the toner marks, and based on timing differences between the sensing of toner marks produced by different toner development systems, registration or alignment of the multiple toner development systems is achieved. The MOB process has the advantages of being automated and amenable to rapid periodic registration calibration during printing. The dedicated MOB sensor and timing-based alignment process substantially increases printing system cost and complexity.
Another approach is disclosed in Parisi et al., U.S. Pat. No. 6,493,083. In this approach, a dedicated optical sensor measures test pattern areas to detect toner development system misalignments. This approach again involves a dedicated sensor, thus increasing printing system cost and complexity.